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From The Collaborative International Dictionary of English v.0.48:
Yeast \Yeast\, n. [OE. [yogh]eest, [yogh]est, AS. gist; akin to D. gest, gist, G. gischt, g[aum]scht, OHG. jesan, jerian, to ferment, G. gischen, g[aum]schen, g[aum]hren, Gr. ? boiled, zei^n to boil, Skr. yas. [root]111.] [1913 Webster] 1. The foam, or troth (top yeast), or the sediment (bottom yeast), of beer or other in fermentation, which contains the yeast plant or its spores, and under certain conditions produces fermentation in saccharine or farinaceous substances; a preparation used for raising dough for bread or cakes, and making it light and puffy; barm; ferment. [1913 Webster] 2. Spume, or foam, of water. [1913 Webster] They melt thy yeast of waves, which mar Alike the Armada's pride, or spoils of Trafalgar. --Byron. [1913 Webster] [1913 Webster] Yeast cake, a mealy cake impregnated with the live germs of the yeast plant, and used as a conveniently transportable substitute for yeast. Yeast plant (Bot.), the vegetable organism, or fungus, of which beer yeast consists. The yeast plant is composed of simple cells, or granules, about one three-thousandth of an inch in diameter, often united into filaments which reproduce by budding, and under certain circumstances by the formation of spores. The name is extended to other ferments of the same genus. See Saccharomyces. Yeast powder, a baling powder, -- used instead of yeast in leavening bread. [1913 Webster]